1. Field of the Invention
The present invention relates to a semiconductor physical-quantity sensor employing a semiconductor substrate and a method of fabricating same, and more particularly, to a semiconductor physical-quantity sensor having a beam structure and employing electrostatic force to detect acceleration, yaw rate, or the like, and a method of fabricating same.
2. Related Arts
A device indicated in publication SAE910496 exists as an acceleration sensor according to the prior art having a thin-film beam structure. FIG. 27A indicates an entirety of this acceleration sensor.
In FIG. 27A, A0 is a Si substrate, A1 is a beam, A2 is a mass, A3 is a movable electrode which forms electrostatic capacitance and performs servo operation, and A4 is a fixed electrode which forms electrostatic capacitance in an interval with A3 and performs servo operation. From A1 to A4 is formed of polycrystalline silicon, and the mass A2 and movable electrode A3 are so supported by the beam A1 as to be disposed away from the Si substrate A0 with a predetermined interval interposed therebetween. Additionally, the beam A1 and fixed electrode A4 are fixed to the Si substrate A0 at an edge portion A5.
These are formed of polycrystalline silicon using surface micromachining technology on a silicon substrate.
FIG. 27B indicates a sectional view taken along line B--B of FIG. 27A, and FIG. 27C indicates a sectional view taken along line C--C of FIG. 27A.
To describe a principle of detection of this sensor with reference to FIG. 27B, a movable electrode A31 exists normally in a center of fixed electrodes A41 and A42 on both sides, and electrostatic capacitances C1 and C2 between the movable electrode A31 and the fixed electrodes A41 and A42 are equal.
Additionally, voltages V1 and V2 are applied between the movable electrode A31 and the fixed electrodes A41 and A42, V1=V2 when acceleration is not applied, and the movable electrode A31 is pulled by equal electrostatic forces from the fixed electrodes A41 and A42.
Herein, when acceleration acts in a horizontal direction of the substrate and the movable electrode A31 is displaced, the distances between the movable electrode A31 and the fixed electrodes A41 and A42 change, and the electrostatic capacitances C1 and C2 become unequal.
At this time, when, for example, the movable electrode A31 is taken as having been displaced toward the fixed electrode A41, the voltage V1 decreases, the voltage V2 increases, and the movable electrode A31 is thereby pulled toward the fixed electrode A42 by electrostatic force so that the electrostatic capacitances become equal.
When the movable electrode A31 assumes a central position and the electrostatic capacitances C1 and C2 become equal, the applied acceleration and generated electrostatic forces are balanced equally, and magnitude of the acceleration can be detected from the voltages V1 and V2 at this time.
However, in a sensor provided with an electrode of beam structure as shown in FIG. 27A, when static electricity has changed due to acceleration and the voltages V1 and V2 have been caused to change in accompaniment thereto, there exists a problem that, when V1 and V2 become high, an electrical potential differential between the fixed electrodes A41 and A42 and the semiconductor substrate A0 also becomes exceedingly large, and dielectric breakdown occurs between the fixed electrodes A41 and A42 and the semiconductor substrate A0.
For example, because the dielectric breakdown voltage of the oxide film is approximately 10 MV/cm, 10 V becomes an upper limit for voltage which can be applied to the fixed electrodes A41 and A42 on an oxide film of approximately 10 nm. Thus, voltage applied to the fixed electrodes A41 and A42 is restricted.
A method which forms a sufficiently thick insulation film on the semiconductor substrate surface and forms an anchor portion on this thick insulation film may also be considered as a countermeasure for a problem such as this, but the distance between the fixed portions A41 and A42 and the semiconductor substrate A0 surface is greatly offset with respect to the distance between the movable portion A31 and the semiconductor substrate A0 surface, and a problem occurs wherein the position of the movable portion cannot be controlled with good accuracy. Additionally, forming a sufficiently thick insulation film on the entirety of the semiconductor substrate surface may also be considered, but a problem occurs wherein a new step to form a hole in this thick insulation film becomes necessary in order to wire the movable portion and the fixed portions.
Furthermore, in a sensor provided with an electrode of beam structure as shown in FIG. 27A, sectional views are as shown in FIGS. 27B and 27C, but for such a sensor there also exists a problem will be described hereinafter. Namely, there exists a problem that, in a case where voltage applied to the fixed electrodes of both sides has changed during occurrence of acceleration, electrostatic force exerted between the fixed electrodes of both sides and the substrate changes and the fixed electrodes are deformed and displaced with respect to the substrate, the opposing surface area of the movable electrode and the fixed electrodes differs at both sides of the movable electrode, and as a result, the generated electrostatic force differs from the design and acceleration detection of good accuracy cannot be performed.